The present invention relates generally to the segment of the field of gas analysis instrumentation that involves the use of solid electrolyte cells to measure gaseous species. More specifically, the invention relates to a novel housing configuration and electronic circuitry for use with a heated solid electrolyte cell. As used herein, the term solid electrolyte cell means a quantity of the solid electrolyte, for example zirconia-yttria ((ZrO.sub.2).sub.(1-x) (Y.sub.2 O.sub.3).sub.x), and porous electrical contacts or electrodes connected thereto, usually made of platinum or other suitable materials.
Gas analyzers having a probe with a solid electrolyte cell to measure gaseous oxygen are well known. See, for example, U.S. Pat. Nos. 3,400,054 and 3,928,161, incorporated herein by reference. Another example is the World Class 3000 Oxygen Analyzer sold by Rosemount Analytical Inc. of Orrville, Ohio, available with replacement cells under part no. 4847B61G01/02/03. A common application for such analyzers is the measurement of gaseous oxygen in a flue or duct such as a smokestack. At a given elevated temperature, the solid electrolyte cell generates an EMF V.sub.cell as a function of the concentration of oxygen (or other gaseous specie) exposed to the cell. V.sub.cell can be approximated by the Nernst equation: EQU V.sub.cell =C+S.multidot.log{P(O.sub.2)/P(O.sub.2).sub.REF },
where C is a cell constant, S is a cell slope which is a function of cell temperature T, and P(O.sub.2) and P(O.sub.2).sub.REF are the oxygen partial pressure at a measurement and reference end, respectively, of the solid electrolyte cell. Actual solid electrolyte cells deviate from the Nernst equation to some extent.
It is also known for such analyzers to include analog or digital electronic circuitry in a housing separate from the probe that measures V.sub.cell and provides an analyzer output indicative of the gaseous species concentration. The probe is configured with a heater and thermocouple, controlled by circuitry in the separate housing, to maintain the solid electrolyte cell at a constant elevated temperature (e.g., 750 C.).
An object of the invention is to provide a compact, rugged, low-cost housing for a solid electrolyte gas analyzer. The housing should simplify installation and provide structural integrity for the analyzer, while maintaining the analyzer circuit at an acceptable operating temperature.
Another object of the invention is to reduce the size and heat dissipation of the analyzer circuit to permit such circuit to be housed in a compact and relatively high temperature compartment.
Still another object of the invention is to provide a heater circuit and preferably a complete analyzer circuit capable of receiving different AC line input voltages (e.g., 120 VAC and 240 VAC) and self-adjusting for such different AC inputs so as to maintain the heater and solid electrolyte cell at a specified temperature.
Other objects of the invention will become apparent from the detailed description of the invention and the accompanying drawings.